Topological phase transition in the interaction of surface Dirac fermions in heterostructures.

Material with a nontrivial topology in its electronic structure enforces the existence of helical Dirac fermionic surface states. We discover emergent topological phases in the stacked structures of topological insulator and band insulator layers where the surface Dirac fermions interact with each other with a particular helicity ordering. Using first-principles calculations and a model Lagrangian, we explicitly demonstrate that such helicity ordering occurs in real materials of ternary chalcogen compounds and determines their topological-insulating phase. Our results reveal the rich collective nature of interacting surface Dirac fermions and pave the way for utilizing topological phases for technological devices such as nonvolatile memories.